An S1 nuclease-sensitive homopurine/homopyrimidine domain in the c-Ki-ras promoter interacts with a nuclear factor.

To gain insight into the normal controls mediating expression of the c-Ki-ras protooncogene, we have identified DNA sequence elements within its promoter that are essential for transcriptional activity. Transient expression assays using the bacterial chloramphenicol acetyltransferase gene were used initially to localize regions directing primary promoter function. Stepwise deletion of 5' promoter sequences resulted in a gradual decrease in the ability to drive transcription of the reporter gene, suggesting that this promoter is composed of multiple cis-acting elements. Gel mobility-shift and DNase protection studies involving a 166-base-pair DNA fragment allowed the identification of protein-binding sites corresponding to these multiple regulatory elements. One element demonstrating particular transcriptional influence exists as a homopurine/homopyrimidine-rich region that in vitro exhibits S1 nuclease sensitivity and binds at least one nuclear protein. Data from competition binding experiments suggest that this nuclear factor may be influential in the regulation of other essential growth-control genes as well.